I have been helping out in a darkroom and I have inherited an unusual way to mix Xtol and no one knows why we mix it the way we do. Here is how we mix it, first we dissolve part "A" in one gallon of warm water, then we add another gallon and dissolve part "B." This of course does not equal a 1+0 or 1+1 ratio. We also have what appears to be an in house developing time chart. Here are some examples of developing times (these times are figured on 20 degree Celsius):
Delta 100 - 11:30
Delta 400 - 11:30
HP5 - 10:00
TriX - 8:30
Tmax400 - 10:00

We generally get good results with these times, but whenever I try to use a film we don't have on this chart I generally don't get good results. So, my question is does anyone have an idea as to why we might have this ratio or just what the ratio might be considered? It seems to be 1+0.5, but no one has any kind reference chart for that ratio. My concern is that if I go to a stock formula or to 1+1 I will have to come up with a whole new chart and I am not really sure how to do that. Anyway thanks in advance for any feedback.